how often do we really see need for 2.5Gbps+ connections?
Among this crowd? More often than in the general population, to be sure.
can your home NAS sustain transfer speeds considerably exceeding 1Gbps (125MBps) for extended periods of time?
There are some that can. There also exist 10GbE units from QNAP and Synology which I'd expect to sustain over a gigabit on certain real-world tests such as bulk media file copies. That's only 125 MB/sec, which we got past for single-disk performance long ago. An 8-wide RAID of four striped mirrors should be able to fill a 10G pipe all by itself with modern disks.
And we aren't even talking about SSD NASes yet.
moving traffic faster than Gbps over copper consumes a lot of energy, causing excesive heating etc.
The proper comparison isn't to 1G, it's to external Thunderbolt enclosures, which also take energy and throw off heat. Nothing's free.
If you want to transfer data over connections longer than a few metres, then bulky UTP Cat7 cables add to the problem.
The TrueNAS Mini X+ in the first link offers SFP+ as an option. It was tested in that configuration, not with the 10GbE ports you get on the base model.
SFP+ ports support modules downward compatible with slower speeds (5Gbps and 2.5Gbps included).
2.5 and 5 Gbit/sec Ethernet is also a thing, largely invented so people could keep using existing Cat-5/e cabling. This so-called NBASE-T is appearing more often now as several of Intel's higher-end chipsets support it.
Only drawback of using SFP+ modules compared to "native" RJ45 ports is lack of PoE.
I feel more comfortable running Cat-6 across the floor where people can step on it than I do with fiber. :)